1. (Field of the Invention)
The present invention relates to a power amplifier for use in communication apparatus operating in microwave or milliwave bands in mobile communication, satellite communication, etc.
2. (Description of the Prior Art)
A Doherty amplifier, which forms a basis for the power amplifier of the present invention, was initially proposed by Mr. W. H. Doherty in “A New High Efficiency Power Amplifier For Modulated Waves”, Proceedings of the Institute of Radio Engineers, Vol. 24, No. 9, September 1936. This known Doherty amplifier was intended for use in a low-frequency band, such as amplitude modulation (AM) broadcasting. Meanwhile, for example, Japanese Patent No. 2945833 discloses a microwave Doherty amplifier used at microwave band by expanding the concept of the known Doherty amplifier. In this Japanese Patent, two second harmonic tuning networks for controlling harmonic load on signal frequency are, respectively, provided at output sides of transistors of a main amplifier and an auxiliary amplifier. However, the Japanese Patent does not teach that the configuration of the second harmonic tuning network of the main amplifier is different from that of the second harmonic tuning network of the auxiliary amplifier.
FIG. 9 shows a conventional microwave Doherty amplifier. The conventional microwave Doherty amplifier includes a main amplifier 110, an auxiliary amplifier 120, a distribution circuit 130, an electrical input side phase adjusting circuit 135 and a Doherty circuit 140. Supposing that “λ” denotes a wavelength corresponding to a signal frequency, the Doherty circuit 140 includes a (λ/4) Doherty network 141. The main amplifier 110 is formed by a transistor 112, an input circuit 111 for performing fundamental wave matching and harmonic processing of an input of the transistor 112 and an output circuit 113 for performing fundamental wave matching and harmonic processing of an output of the transistor 112. To this end, the input circuit 111 includes an input matching circuit 111A and an inverse Class F harmonic processing circuit 111B, while the output circuit 113 includes an output matching circuit 113A and a harmonic processing circuit 113B. Meanwhile, the auxiliary amplifier 120 is formed by a transistor 122, an input circuit 121 for performing fundamental wave matching and harmonic processing of an input of the transistor 122 and an output circuit 123 for performing fundamental wave matching and harmonic processing of an output of the transistor 122. To this end, the input circuit 121 includes an input matching circuit 121A and a Class F harmonic processing circuit 121B, while the output circuit 123 includes an output matching circuit 123A and a harmonic processing circuit 123B.
In the conventional microwave Doherty amplifier of FIG. 9, since harmonic processing conditions defined by the inverse Class F harmonic processing circuit 111B and the harmonic processing circuit 113B which perform harmonic processing of the main amplifier 110 are identical with those defined by the Class F harmonic processing circuit 121B and the harmonic processing circuit 123B which perform harmonic processing of the auxiliary amplifier 120, both the main amplifier 110 and the auxiliary amplifier 120 operate in Class F. Furthermore, a circuit for setting different harmonic processing conditions in the main amplifier 110 and the auxiliary amplifier 120 is not provided in the conventional microwave Doherty amplifier of FIG. 9.
In the prior art microwave Doherty amplifier of the above Japanese Patent in which the two second harmonic tuning networks having an identical configuration are, respectively, provided at the output sides of the transistors of the main amplifier and the auxiliary amplifier so as to act as harmonic load control circuits and the conventional microwave Doherty amplifier of FIG. 9 in which the main amplifier 110 and the auxiliary amplifier 120 have identical harmonic processing conditions, it is difficult to obtain high-efficiency characteristics.
Thus, through rigid investigations, the present inventor has found that if a configuration of the second harmonic tuning network of the main amplifier is made different from that of the second harmonic tuning network of the auxiliary amplifier in the prior art microwave Doherty amplifier of the Japanese Patent referred to above, it is possible to obtain high-efficiency characteristics. Moreover, the present inventor has confirmed that if different harmonic processing conditions are set in the main amplifier 110 and the auxiliary amplifier 120 of the conventional microwave Doherty amplifier of FIG. 9, higher efficiency can be gained.